1. Field of the Invention
The present invention relates to an image pickup device capable of switching to and from a still image photographing and a moving image recording, more particularly to an image pickup device having an entire pixel reading mode in which pixel data read from a photoelectric conversion element array corresponding to entire pixels is sequentially outputted by one pixel each at the time of the still image photographing and a vertical/horizontal pixel mixing reading mode in which pixel data corresponding to a plurality of pixels is mixed in vertical and horizontal directions of the array and outputted at the time of the moving image recording.
2. Description of the Related Art
The number of pixels in a photoelectric conversion element array in an image pickup device has been significantly increasing, which leads to a higher pixel density. In photographing a still image, data of all of pixels in the photoelectric conversion element array are used, which is referred to as an entire pixel reading mode. In the entire pixel reading mode, a high-resolution still image can be photographed because pixel data read from the photoelectric conversion element array corresponding to the entire pixels are sequentially read by one pixel each.
An image pickup device capable of switching to and from a still image photographing and a moving image recording has been proposed (for example, see No. 2002-135793 and No. 2003-116061 of the Publication of the Unexamined Japanese Patent Applications). Currently, it is difficult to record the moving image in the entire pixel reading mode in the same manner as in the still image photographing. The moving image is recorded by thinning the pixels/vertically and horizontally mixing the pixels.
In recording the moving image, the pixel data corresponding to a plurality of pixels read from the photoelectric conversion element array are vertically and horizontally mixed, and the mixed pixel data is outputted in the form of pixel data of one unit. Thereby, the number of frames per unit time increases, which enables the moving image to be smoothly photographed at a high speed even in the image pickup device in which the photoelectric conversion element array of the high pixel density.
The selection between the pixel thinning/pixel mixing reading operation and the entire pixel reading operation is advantageously handled by, in particular, an MOS (Metal Oxide Semiconductor) image sensor because the MOS image sensor does not require charge transfer in response to the movement of a potential well as in a CCD (Charge Coupled Device) image sensor and can read the pixel data in an optional line without any restriction using a signal line. The MOS image sensor is advantageous in reading an optional number of pixels and mixing the pixels in addition to a low voltage operation, a small amount of lead current, a large opening rate, a high sensitivity, a simplified data reading operation and the like.
In an image pickup device provided with a photographing monitor such as a liquid crystal display device is performed an optical-system auto focus. In general, an image of a photographic subject is displayed on the photographing monitor in a moving image mode in an initial state immediately after a power supply is turned on, which is a generally-called monitor mode. In the monitor mode, which is employed in handling the moving image, a vertical/horizontal pixel mixing reading mode in which the pixels are thinned is adopted. Pressing a shutter button in the monitor mode in order to photograph the still image, the auto focus of the optical system is activated when the shutter button remains pressed halfway, the shutter is fully pressed (fully-pressed state) in a focusing-attained state. Then, the current mode is switched to the entire pixel reading mode so as to photograph the still image.
An example of a conventional technology relating to the auto focus is described referring to FIG. 12. In Step S31, an operating state is a photographing standby state immediately after a power supply is turned on, and an operating mode is set to the monitor mode. In Step S32, the mode is set to the vertical/horizontal pixel mixing reading mode. In Step S33, it is judged if the moving image is recorded or the still image is photographed. The operation proceeds to Step S34 when the moving image recording mode is selected by a photographer, and to Step S36 when the still image photographing mode is selected by the photographer.
In the Step S34 in which the moving image recording mode is selected, a lens of the optical system is driven in response to the activation of the auto focus. In Step S35, the moving image is recorded in the vertical/horizontal pixel mixing reading mode when a recording button is manipulated. In the Step S34, the auto focus based on the pixel data mixed in the vertically and horizontally two-dimensional directions is performed.
In Step S36 in which the still image photographing mode is selected, the lens of the optical system is driven in response to a rough auto focus. In Step S37, the halfway-pressed state of the shutter button is judged. In Step S38, the auto focus based on the pixel data mixed in the vertically and horizontally two-dimensional directions is activated, and the lens of the optical system is thereby driven in the same manner as in the Step S34. When it is judged the focusing is attained in Step S39, the operation proceeds to Step S40, in which the mode is set to the entire pixel reading mode, and the shutter button is allowed to be fully pressed. Thereby, the still image is photographed in the entire pixel reading mode.
In the foregoing operation, the auto focus in the Step S34 is performed in the state in which a high frequency component exceeding a pixel mixing pitch of the image sensor is included in an optical image having transmitted through the lens.
In the conventional auto focus, a high-pass component in a frequency characteristic of a signal gain of the pixel data is extracted through a band pass filter, and it is generally determined that the focusing is attained at a peak value of the high-pass component. FIG. 13A shows a frequency characteristic of imaging data when three pixels by every other pixel are mixed in the horizontal direction or vertical direction. In FIG. 13A, a characteristic line L1 denotes the pixel mixing, a characteristic line L2 denotes no pixel mixing, and a characteristic line L3 denotes an optical frequency characteristic (f characteristic) of a lens. FIG. 13B shows a characteristic of a highpass horizontal band pass filter (BPF) used in the auto focus at the time of the moving image mode and still image mode.
When the pixels are mixed and thinned, the Nyquist frequency shifts to a lowpass side, and horizontal highpass information is folded back toward the lowpass side. As a result, in the conventional technology in which the vertical/horizontal pixel mixing reading mode is adopted when the auto focus is controlled in photographing the moving image, the auto focus is controlled in the state in which the optical image having transmitted through the lens includes the high frequency component exceeding the pixel mixing pitch of the image sensor, that is the state in which the high frequency component of the optical image has been folded back in a spatial sampling on the image sensor. Therefore, a false signal due to the folded-back component is generated in the focused state, which deteriorates an image quality.